The invention relates to a method of controlling the position of a write-read head(s), which cooperate(s) with a record carrier on which consecutively adjacent parallel information tracks are to be written.
In systems in which information is recorded in adjacent tracks and is subsequently read, in particular systems which employ a magnetic record carrier in the form of a tape on which for example video signals are recorded and subsequently read and in which the tracks make an (generally small) angle with the longitudinal axis of the record carrier, it is essential that during reading the write-read head accurately follows the desired track. This is essential since, in order to increase the information density, increasingly smaller track pitches are selected and the tracks are written directly adjacent each other, i.e. without intermediate space, while at the same time, the width of the tracks is reduced continually. In order to ensure accurate tracking during the reproduction of the information, various control systems have been proposed, which control the relative position of the read head with respect to the record carrier transverse to the tracks during reproduction.
However, it has been found that it is equally essential to ensure that, during recording of information on such record carriers, the recorded tracks exhibit a most uniform pattern, i.e. that the track pitches are constant to the highest possible degree. This is of particular importance for the compatibility of the record carriers, i.e. the possibility of reproducing information recorded on a record carrier by means of an apparatus other than that with which it has been recorded. Furthermore, this becomes more important as the desired spacing between the tracks is reduced, because then there is a risk that the information signals of two consecutive tracks are fully or partly superimposed.
It is therefore also desirable to control the position of the write-read head transverse to the track direction during recording of information on the record carrier, so as to obtain the desired uniform track configuration as exactly as possible. This is especially desirable if for the positional control of the write-read heads during playback the write-read heads are not mounted rigidly in the direction transverse to the tracks, because then the position of these heads in this direction is not accurately defined during recording.
A method of obtaining this positional control of the write-read head during the recording of information on such a record carrier has been described previously in U.S. Pat. No. 3,845,500. The system described in this patent employs an additional servo track at the edge of the tape-like record carrier, in which servo track synchronizing marks are recorded. During the recording of information on the record carrier, the write-read head first scans this servo track, i.e. the synchronizing mark associated with the information track to be written, a control signal being derived from the detected relative position between the write-read head and said synchronizing mark, which control signal controls the position of the write-read head at the beginning of the information track.
Thus, this known system requires the presence of at least one servo track at the edge of the record carrier, which is obviously at the expense of the amount of information that can be recorded on the record carrier. Moreover, this system makes it necessary to record the synchronizing marks in the servo track before information can be recorded in the information tracks. Finally, this system requires a considerable amount of additional high-precision electronic circuitry.